FI72401C - ELECTRICAL INSULATION MATERIAL. - Google Patents

Abstract

The invention relates to an electrical insulation composition of a polyolefin, which can be used for medium and high voltage from about 10 kV and which resists effectively and permanently the formation of so-called water trees. The insulative polyolefin contains an inorganic oxide additive, in homogeneous distribution therewith, at a concentration of 0.05 to 10% by weight, relative to the total weight of the composition. The additive is absorption-active for heavy metal ions and/or binds heavy metal ions in an ion exchange, with a particle size of up to 50 um or an agglomerate size of up to 100 um. It can be silica, alumina, hydrated alumina or an aluminum silicate. The electrical insulation according to the invention is particularly well-suited for use in cables and wires.

Description

1 72401 Electrical insulating materials

The invention relates to polyolefin-based electrical insulators, in particular for cables and conductors for medium and high voltages from about 10 kV, which insulators contain an additive to prevent water branching.

Electrically stressed polyolefin insulators can have phenomena known as "Electrochemical treeing" (ECT) or "water trees" (water branching). These phenomena are particularly important for plastic-insulated medium and high voltage cables. from the point of view of operational safety, lead to the formation of woody structures, so-called ECT structures, which, after suitable staining, show particularly high contrasts and detail, have a multifaceted optical manifestation, basically distinguishing between two forms: - "vented trees", which leave the insulation surface 20 and extend into the insulation, and - "bow-tie trees" which arise inside the insulation.The mechanism of ECT formation has not been elucidated so far, however, it is generally assumed that an electric field and the presence of polar fluid, especially water, are required to form ECT structures. therefore ECT structures are also referred to as v esihaaroittumiksi. The origins of water branches always appear to be disturbance sites, such as contaminants, accumulated additives, cavities, cracks, tears, or interfaces, however, only 30 parts of each time lead to the formation of water branches. Areas of interference that cannot be completely avoided in large-scale insulators are wooden structures extending in the direction of the electric field.

Because ECT structures are local changes in the secretion material, they can cause damage to the insulation, particularly with respect to electrical breakdown strength. Therefore, several secretions have already been made to prevent or at least slow down the growth of water branches.

One of the measures to prevent the formation of water branches is based on the fact that the release layer is provided with 5 metal cladding or sheathing, which is, for example, lead or aluminum. Cables equipped with such a waterproofing layer are not only more expensive, but also heavier and thus also more difficult to handle than cables without a metal sheath.

1Q For this reason, it has already been secreted to prevent the formation of ECT structures by adding additives to the insulating layer (or to the limiting layers). Examples of the numerous compounds used in this case are: lead stearate (German Publication No. 15 DE ^ OS 24 25 760 or British patent GB-PS 1 473 867). sodium chloride and sulphate or other strong electrolytes (German Offenlegungsschrift DE-AS 25 37 283 or U.S. Pat. No. 4,042,776), salts forming stable hydrates, such as calcium and magnesium chloride, and basic anhydrides 20 (DE-OS 28 17 804 or GB-PS 1 584 501), silica gel and phosphorus pentoxide (DE-OS 27 54 336), organosilanes (U.S. Patents 4,144,202, 4,212,756 and 4,263,158 and DE-OS 28 05 875), lead oxides and basic lead compounds (DE-OS 25 23 844 and DE-OS 28 06 752), organic isocyanates (U.S. Pat. No. 25,428,333), polymer-grafted silane compounds (DE-OS 29 35 224) and diketonic metal complexes , salicylic acid and Schiff bases (EP - Ai - 27 300).

On the other hand, and contrary to the proposals to add salt-like compounds or electrolytes, it has already been proposed to keep the finely divided water-soluble and / or hygroscopic salts in the insulator below -1 -4 10 ppm and preferably below 10 ppm (DE-OS 29 11 756).

Apparently, not all of these measures, which are also partly contradictory, did not lead to the desired

II

3 72401. Thus, a report from Cigrg, i.e. the International High Voltage Conference of 1980, states (see "Kunststof £ e" 71, 1981, pages 448-) that it was recommended to place a water-5 metal shield on (high voltage) cables to prevent moisture penetration.

The object of the present invention is to effectively or permanently prevent or prevent the formation of water branches with suitable additives by means of the above-mentioned electrical insulators, i.e. based on crosslinked and non-crosslinked polyolefins, so that costly measures such as metal sheaths can be dispensed with.

According to the invention, this is achieved in that the electrical insulators as an additive contain, in a homogeneously distributed form, from 0.05 to 10% by weight, based on the total weight, of a heavy metal ion adsorbent or heavy metal ion ion-binding substance is at most 100 μm. The electrical insulators may then contain additives individually or in the form of mixtures.

By "homogeneous distribution" in the context of this patent application is meant that no special measures are necessary or required to affect the future entanglement in the machining process.

Preferably, the electrical insulators according to the invention contain pyrogenic and / or precipitated silicic acid as additives. However, alumina and oxide hydrates with a high active surface area and / or aluminosilicates can also be advantageously used as additives. In this case, synthetic products are preferably used as additives. However, naturally occurring minerals may also be used, and in particular when they meet high requirements for purity and uniformity of particle size; the ki-35 brothers must then contain as little as 4,72401 side stones and as few traces of impurities as possible.

But also in general, the additives should be as pure as possible.

The silicas contained in the electrical insulating materials according to the invention are preferably synthetic products in a very finely divided form, these silicas (chemical composition: SiO 2 x I 2 O) being further characterized in that they are not crystalline but X-ray amorphous. Synthetic silicas are generally prepared by pyrogenic or wet methods. Pyrogenic silicic acid is formed, for example, by hydrolysis of silicon tetrachloride in an explosive flame (temperature: about 1000 ° C). Precipitated silicic acid (i.e., precipitated silicic acid) is prepared from sodium silicate solutions by the addition of acids such as hydrochloric or sulfuric acid; The precipitated product is then washed, dried and heated to 300-800 ° C, preferably 400-700 ° C and then ground.

The aluminas and oxide hydrates contained in the electrical insulating materials can be used in the form of naturally occurring minerals such as AlO (OH) or AlO (O).

However, these oxides and oxide hydrates can also be synthesized - by known methods. For this purpose, for example, oxide hydrates are mixed from aluminum sulphate solutions with ammonia and then dried and heated to temperatures of 400-750 ° C, preferably 500-600 ° C and then ground.

Aluminosilicates can also be used in the form of (purified) natural mineral products, such as kaolinite Al 2 O (OH) g / SiO 2 / or Al 2 O 2. 2 S1O2: 2 H2O, montmorillo-30 rivet in the form of Al2 (OH) 2 / Si2O2, J2 or Al2O2, SiO2.H2O and bentonite, a clay mineral in which montmorillonite is the main constituent. However, aluminosilicates can also be precipitated from solutions by the same known methods; after drying, they are then heated to 350-950 ° C, preferably 450-700 ° C and then ground.

5,72401

The additives according to the invention can, on the one hand, prevent the formation of ECT in electrical insulators and, on the other hand, expand the supply of active additives due to these compounds.

5 To date, this supply includes barbituric acid or 2-thiobarbituric acid and its derivatives (German Public Procurement Publication 32 02 828) and water-soluble alkali * or alkaline earth phosphates and hydrolysable phosphoric acid esters (German Public Publication Publication 10 32 02 89.6).

DE-OS 27 54 336 already discloses a high-voltage cable with plastic, in particular polyolefin, conductor insulation, in which the cable has a diffuse gradient from the inside to the outside in the plastic insulation. This measure should make it possible to prevent the formation and growth of small water branches in the extruded conductor insulation as reliably as possible. In order to provide a diffusion gradient to the conductor insulation, a number of possibilities are then provided, such as surrounding the conductor insulator 20 with a layer of water-absorbent material or a material with a high loss coefficient, in the insulation of the water-absorbent material with increasing material content from the inside to the outside (such as silica gel and or heating the outer shield surrounding the iodine-25 insulation and conducting a dry gas flow between the conductor insulation and the outer sheath; in this case, several of these measures should preferably be used simultaneously: nothing has been said regarding the concentration and particle size of the silica gel and phosphorus pentoxide; moreover, the use of such water-absorbing substances also does not provide any permanent solution to the problem.

However, all these measures mentioned are very expensive; this also applies to providing a concentration gradient in the insulating layer. Therefore, it was more than surprising and it was also unpredictable that - also to achieve high efficiency 6,72401 - it is sufficient, for example, to use pyrogenic or precipitated silicic acid as an additive to prevent ECT 2 formation and to distribute this additive homogeneously. This also applies to DE-OS 28 05 275, as mentioned above, in DE-OS 28 05 275. It is known to add field-limiting layers of high-voltage cables containing conductive carbon black, in particular external conductor layers, to prevent the formation of ECT. and adsorption-active fillers in the form of surfactant soot, finely divided precipitated silicic acid and / or synthetic silicic acid. However, the essential additive in this case is alkoxysilane, which is hydrolysed by the water penetrating the cable, the so-called its function 15 is to chemically bind the penetrating water. Namely, the aim of said measures is to reduce the moisture content in the insulation. These metal ions are bound to bind the additives according to the invention and the moisture present in the secretion is no longer able to cause ECT formation.

In addition to cables and conductors, the electrical insulating materials according to the invention can also be used in sleeves and devices. These secretory materials are based on polyolefins and crosslinked or non-crosslinked materials.

In particular, polyethylene (PE) and crosslinked polyethylene (VPE) can be used in the insulators according to the invention. However, ethylene copolymers such as ethylene-propylene copolymers (EPR), ethylene-vinyl acetate copolymers (EVA) and ethylene-alkyl acrylate copolymers (e.g. ethylene-ethyl acrylate and butyl acrylate-copolymers) or ethylene copolymers can also be used. propylene-diene terpolymers and 7,72401 blends of these ethylene copolymers and terpolymers (blendsX with polyolefins, in particular polyethylene and polypropylene. Said polymers or polymer blends can, as already mentioned, be used both crosslinked and non-crosslinked, in which case the crosslinking takes place. energy-rich rays Special materials may also be provided with antioxidants.

The proportion of additives is, as already mentioned, 0.05 to 10% by weight, based on the total weight of the electrical insulating material. Preferably the proportion of additives is between 0.1 and 4% by weight, in particular between 0.5 and 2% by weight. As before, the content of additives is limited so that the dielectric loss factor does not increase significantly. In addition, the specifications of IEC publication 502 (1978 edition) must be observed.

Additives are added to the insulation. However, in cables and conductors, additives - in addition to the actual insulating layer - can also be added to the field-limiting layers, i.e. to the inner and / or outer conductor layer.

The electrical insulating materials according to the invention have an additional particle size of at most 50 [mu] m or an agglomerate size of at most 100 [mu] m. By agglomerates is meant fluffy agglomerations of particles which - for example by shear forces - can be decomposed in a larger or smaller amount. In particular, the particle size is at most 20.

Additives can also be mixed with polyolefins together in the so-called with retardants, with the paste formed by the additive and the retardant being preferred. Paraffins, paraffin alcohols and esters are particularly suitable retardants, with hydrocarbons, i.e. non-polar compounds, being preferred for the preservation of electrical properties.

The invention is further illustrated by exemplary embodiments.

To demonstrate the reduction in water branching, high purity, unstabilized 35 thermoplastic high pressure (LDPE) polyethylene sheets of 2 mm thick were prepared - with and without additive. In order to prepare the insulating agents according to the invention, a portion of the sheet material was then homogeneously mixed with intra-pyrogenic silicic acids having 5 different active surfaces and commercially available as Aerosil® in concentrations of 0.5-1% by weight. The plate-like specimens were then electrically loaded with 10 kV / 50 Hz, 11a, with both surfaces in direct contact with a 3% sodium chloride-10 solution heated to 70 ° C. The loading time was 130 hours.

The test results show that the test plates containing pyrogenic silicic acid - under the same experimental conditions - compared to the test pieces without the addition of silicic acid contain less or even no ECT structures. Furthermore, there is also a significant difference in the size of ECT structures. Namely, while the elongation of the ECT structures in the direction of the electric field in the test pieces without the addition of silicic acid is up to 1500 μm, the ECT structures of the silica-containing test pieces are less than 50 μm.

20 The following, for example, will appear in detail

Additive Relative length of ECT structures Number of ECT structures (compared to comparative test) 25 - (comparative test) Z. 1500 yUm

Aerosil 130 500 ^ um smaller "200 300ι 300 um much smaller" 300 - not at all "380 - not at all

3Q

The pyrogenic silicas described above have, in the order shown, an increasing specific surface area, measured by the BET method (Aerosil 130: 130 + 25 m 2 / g...

2

Aerosil 380: 380 + 30 m / g), It is clear from this that in the electrical insulating materials according to the invention 35 the additives preferably have a high specific surface area.

Pyrogenic silicic acids have similar results to those obtained with the addition of precipitated silicic acid (hydrophilic, BET surface area; 170 m / g), namely, while 5 flat test pieces without additive, as already mentioned, have numerous ECT structures with a length of Up to 1500 μm, with the addition of silica precipitated on test specimens; 0.5-1% by weight) very few ECT structures with a length of 100 .mu.m.

The electrical insulating agents according to the invention are characterized in that the additives are hydrophilic, as is the case, for example, with synthetic silicic acids. Namely, these silicic acids have SiOH groups on the surface. If these groups are replaced, for example, by reaction with alkylchlorosilanes, then partially or completely hydrophobicized products are formed. One such product is, for example, as an additive to electrical insulators, so - under the conditions described above - as many ECT structures are obtained as in the test pieces without the additive, but the length of the ECT structures is even greater: up to 2400 μm.

The defendant applies to hydrophobic precipitated silicic acids. The test specimens with such silicic acids 2 (BET surface area: 100 m / g) then even have an increased number of ECT structures compared to the comparative test, and in addition their length is even greater (^ 1900 μm).

Similar results to those obtained with silicic acids are also obtained with the addition of aluminas or oxide hydrates and aluminosilicates (content: 0.1 to 1% by weight). Thus, for example, test specimens to which kaolinite 2 3Q or metakaolinite (BET surface area: 70 m / g) have been added have few ECT structures with a length of more than 500 μm. and a test specimen to which a synthetic V "-AlO 2 ^ (BET surface area: 2 ^ 220 m / g few ECT structures with a length of 300 μm) has been added.

The results obtained with the plate test pieces are confirmed by studies performed with cables. Thus, for example, 10,72401 test cables with 2% by weight of Aerosil 300 added to the insulator gave - after 2600 hours of loading - only random ECT structures up to 200 .mu.m in length, while the test cable without addition had a large 5 EC]. > structures up to 100 μm in length.

Thus, it is found that the formation of ECT can be considerably reduced by the additions according to the invention and this already at low concentrations. This has the further advantage that the electrical properties of the insulation materials are not affected at all or only insignificantly.

Claims (7)

11 72401 1. Polyolefin-based electrical insulators, in particular for cables and conductors, for medium and high voltages from approx. 10 kV, in which an additive 5 has been used to prevent water trees, characterized in that they contain 0 as an additive, homogeneously distributed, 05-10% by weight, based on the total weight, of a substance which adsorbs heavy metal ions or binds heavy metal ions in an ion exchange with a particle size of not more than 50 μm or an agglomerate size of not more than 100 yarn. / Electrical insulating materials according to Claim 1, characterized in that they contain pyrogen as an additive. and / or precipitated silicic acid. Electrical insulating materials according to Claim 1, characterized in that they contain high-surface area aluminas and oxide hydrates and / or aluminosilicates as additives. Electrical insulators according to one of Claims 1 to 3, characterized in that they contain additives in the form of synthetic products. Electrical insulating materials according to one or more of Claims 1 to 4, characterized in that the additive is present in an amount of 0.1 to 4% by weight, in particular 0.5 to 2% by weight. Electrical insulating agents according to one or more of Claims 1 to 5, characterized in that the additives are mixed into the polyolefins together with retardants, preferably in the form of a paste. Process for the preparation of electrical insulating material according to one of Claims 1 to 6, characterized in that the additives are mixed into the polyolefins together with retardants, preferably in the form of Dasta.